Semiconductor device and method of manufacturing semiconductor device

ABSTRACT

A semiconductor device includes a first semiconductor layer made of a nitride semiconductor and formed on a substrate, a second semiconductor layer made of a material including InAlN and formed on the first semiconductor layer, an insulator layer formed by an oxidized surface part of the second semiconductor layer, a gate electrode formed on the insulator layer, and a source electrode and a drain electrode respectively formed on the first or second semiconductor layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priorities of the prior Japanese Patent Applications No. 2014-085621, filed on Apr. 17, 2014, and No. 2015-040411, filed on Mar. 2, 2015, the entire contents of which are incorporated herein by reference.

FIELD

The embodiments discussed herein are related to a semiconductor device and a method of manufacturing the semiconductor device.

BACKGROUND

Materials such as nitride semiconductors, including GaN, AlN, and InN, and mixed crystals of such nitride semiconductors, have a wide band gap. Hence, such materials having the wide band gap are used for high-output electronic devices, short wavelength light emitting devices, or the like. For example, GaN, which is an example of the nitride semiconductor, has a band gap of 3.4 eV that is wider than a band gap of Si that is 1.1 eV and also wider than a band gap of GaAs that is 1.4 eV.

An FED (Field Effect Transistor), and particularly an HEMT (High Electron Mobility Transistor) as proposed in Japanese Laid-Open Patent Publication No. 2002-359256, for example, is an example of the high-output electronic device. The HEMT using the nitride semiconductor may be used for a high-output and high-efficiency amplifier, a high-power switching device, or the like. More particularly, in the HEMT that uses AlGaN for an electron supply layer and GaN for an electron transit layer, a piezoelectric polarization or the like may occur in AlGaN due to a distortion caused by a difference between lattice constants of AlGaN and GaN, to thereby generate 2DEG (2-Dimensional Electron Gas) having a high concentration. For this reason, this HEMT may operate at a high voltage, and may be used in a high-efficiency switching device, a high voltage power device for an electric vehicle, or the like.

In some ultra-high-frequency devices using the nitride semiconductor, InAlN having a high spontaneous polarization is used for the electron supply layer in place of AlGaN, in order to obtain a high output from the device. Even a thin InAlN layer may induce the 2DEG having a high concentration, and thus, InAlN is being regarded as a material enabling both high output properties and high frequency properties.

However, in the HEMT that uses InAlN for the electron supply layer, the high spontaneous polarization of InAlN increases an electric field intensity within the electron supply layer. Particularly in the case of an HEMT having a Schottky gate electrode, there is a problem in that a gate leak current increases.

Accordingly, it is desirable to reduce the gate leak current in the HEMT that uses InAlN for the electron supply layer and has the Schottky gate electrode.

SUMMARY

According to one aspect of the embodiments, a semiconductor device includes a first semiconductor layer made of a nitride semiconductor and formed on a substrate; a second semiconductor layer made of a material including InAlN and formed on the first semiconductor layer; a first insulator layer formed by an oxidized surface part of the second semiconductor layer; a gate electrode formed on the first insulator layer; and a source electrode and a drain electrode respectively formed on the first or second semiconductor layer.

The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a structure diagram illustrating a semiconductor device using InAlN for an electron supply layer;

FIG. 2 is a diagram illustrating a correlation between a gate-drain voltage and a gate leak current of the semiconductor device illustrated in FIG. 1;

FIG. 3 is a structure diagram illustrating a semiconductor device in a first embodiment;

FIGS. 4A and 4B are diagrams illustrating analysis results of an insulator layer analyzed by XPS (X-ray Photoelectron Spectroscopy);

FIG. 5 is a diagram illustrating a correlation between oxidation methods and an Al/In ratio when forming the insulator layer;

FIG. 6 is a diagram illustrating a correlation between a gate voltage and the gate leak current of the semiconductor device in the first embodiment;

FIG. 7 is a diagram illustrating a correlation between the gate voltage and the gate leak current of the semiconductor device illustrated in FIG. 1;

FIGS. 8A, 8B, and 8C are diagrams for explaining processes of a method of manufacturing the semiconductor device in the first embodiment;

FIGS. 9A, 9B, and 9C are diagrams for explaining processes of the method of manufacturing the semiconductor device in the first embodiment;

FIGS. 10A, 10B, and 10C are diagrams for explaining processes of the method of manufacturing the semiconductor device in the first embodiment;

FIGS. 11A, 11B, and 11C are diagrams for explaining processes of the method of manufacturing the semiconductor device in the first embodiment;

FIGS. 12A, 12B, and 12C are diagrams for explaining processes of the method of manufacturing the semiconductor device in the first embodiment;

FIGS. 13A, 13B, and 13C are diagrams for explaining processes of the method of manufacturing the semiconductor device in the first embodiment;

FIG. 14 is a structure diagram illustrating the semiconductor device in a second embodiment;

FIG. 15 is a diagram for explaining the semiconductor device in the second embodiment;

FIG. 16 is a diagram for explaining properties of the semiconductor devices in the first embodiment and the second embodiment;

FIGS. 17A, 17B, and 17C are diagrams for explaining processes of the method of manufacturing the semiconductor device in the second embodiment;

FIGS. 18A, 18B, and 18C are diagrams for explaining processes of the method of manufacturing the semiconductor device in the second embodiment;

FIGS. 19A, 19B, and 19C are diagrams for explaining processes of the method of manufacturing the semiconductor device in the second embodiment;

FIGS. 20A, 20B, and 20C are diagrams for explaining processes of the method of manufacturing the semiconductor device in the second embodiment;

FIGS. 21A, 21B, and 21C are diagrams for explaining processes of the method of manufacturing the semiconductor device in the second embodiment;

FIGS. 22A, 22B, and 22C are diagrams for explaining processes of the method of manufacturing the semiconductor device in the second embodiment;

FIG. 23 is a diagram for explaining the semiconductor device in a third embodiment;

FIG. 24 is a circuit diagram illustrating a PFC (Power Factor Correction) circuit in the third embodiment;

FIG. 25 is a circuit diagram illustrating a power supply unit in the third embodiment;

FIG. 26 is a structure diagram illustrating a high-output amplifier in the third embodiment;

FIG. 27 is a structure diagram illustrating a semiconductor device provided with a gate insulator layer;

FIG. 28 is a structure diagram illustrating a semiconductor device in a fourth embodiment;

FIGS. 29A and 29B are diagrams illustrating analysis results of an oxide layer analyzed by XPS;

FIG. 30 is a diagram illustrating a correlation between oxidation methods and the Al/In ratio when forming the oxide layer;

FIG. 31 is a diagram illustrating a composition distribution of the oxide layer of the semiconductor device in the fourth embodiment;

FIG. 32 is a correlation diagram of a gate voltage and a leak current of the semiconductor device in the fourth embodiment;

FIG. 33 is a correlation diagram of the gate voltage and the leak current of the semiconductor device illustrated in FIG. 1;

FIG. 34 is a correlation diagram of a drain voltage and a drain current of the semiconductor device in the fourth embodiment;

FIG. 35 is a correlation diagram of the drain voltage and the drain current of the semiconductor device illustrated in FIG. 27;

FIGS. 36A, 36B, and 36C are diagrams for explaining processes of a method of manufacturing the semiconductor device in the fourth embodiment;

FIGS. 37A, 37B, and 37C are diagrams for explaining processes of the method of manufacturing the semiconductor device in the fourth embodiment;

FIGS. 38A, 38B, and 38C are diagrams for explaining processes of the method of manufacturing the semiconductor device in the fourth embodiment;

FIGS. 39A, 39B, and 39C are diagrams for explaining processes of the method of manufacturing the semiconductor device in the fourth embodiment;

FIGS. 40A, 40B, and 40C are diagrams for explaining processes of the method of manufacturing the semiconductor device in the fourth embodiment;

FIG. 41 is a diagram for explaining the process of the method of manufacturing the semiconductor device in the fourth embodiment;

FIG. 42 is a correlation diagram of the gate voltage and the leak current of the semiconductor device in a fifth embodiment;

FIG. 43 is a correlation diagram of the drain voltage and the drain current of the semiconductor device in the fifth embodiment;

FIG. 44 is a structure diagram illustrating the semiconductor device in a sixth embodiment;

FIG. 45 is a diagram for explaining the semiconductor device in the sixth embodiment;

FIG. 46 is a correlation diagram of the gate voltage and the leak current of the semiconductor device in the sixth embodiment;

FIG. 47 is a diagram for explaining properties of the semiconductor devices in the fourth embodiment and the sixth embodiment;

FIGS. 48A, 48B, and 48C are diagrams for explaining processes of a method of manufacturing the semiconductor device in the sixth embodiment;

FIGS. 49A, 49B, and 49C are diagrams for explaining processes of the method of manufacturing the semiconductor device in the sixth embodiment;

FIGS. 50A, 50B, and 50C are diagrams for explaining processes of the method of manufacturing the semiconductor device in the sixth embodiment;

FIGS. 51A, 51B, and 51C are diagrams for explaining processes of the method of manufacturing the semiconductor device in the sixth embodiment;

FIGS. 52A, 52B, and 52C are diagrams for explaining processes of the method of manufacturing the semiconductor device in the sixth embodiment; and

FIG. 53 is a diagram for explaining the process of the method of manufacturing the semiconductor device in the sixth embodiment.

DESCRIPTION OF EMBODIMENTS

Preferred embodiments of the present invention will be described with reference to the accompanying drawings.

A description will now be given of the semiconductor device and the method of manufacturing the semiconductor device, in each embodiment according to the present invention. In the figures, those parts that are the same are designated by the same reference numerals, and a repetition of the same parts will be omitted.

First Embodiment

First, a description will be given of a semiconductor device using InAlN for an electron supply layer, by referring to FIG. 1. FIG. 1 is a structure diagram illustrating the semiconductor device using InAlN for the electron supply layer. The semiconductor device illustrated in FIG. 1 includes a buffer layer (not illustrated), an i-GaN electron transit layer 921, an AlN intermediate layer 922, and an InAlN electron supply layer 923 that are stacked on a substrate 910. A gate electrode 931, a source electrode 932, and a drain electrode 933 are provided on the electron supply layer 923. A protection layer 940 made of SiN or the like is formed on an exposed region of the electron supply layer 923, not provided with the gate electrode 931, the source electrode 932, and the drain electrode 933. The substrate 910 is made of semi-insulating SiC, and a 2DEG 921 a is generated in the electron transit layer 921 at a vicinity of an interface between the electron transit layer 921 and the intermediate layer 922.

In the semiconductor device having the structure illustrated in FIG. 1, when a 0 V potential is applied to the gate electrode 931 and the drain electrode 933 is set to a high potential, a gate leak current I_(g)leak may flow as indicated by a dotted arrow in FIG. 1. This gate leak current I_(g)leak increases exponentially with an increase of a gate-drain voltage V_(gd), as illustrated in FIG. 2. FIG. 2 is a diagram illustrating a correlation between the gate-drain voltage and the gate leak current of the semiconductor device illustrated in FIG. 1. More particularly, when the gate-drain voltage V_(gd), which becomes a gate reverse voltage applied across the gate electrode 931 and the drain electrode 933, exceeds approximately 20 V, the gate leak current I_(g)leak sharply increases exponentially. For this reason, the gate-drain voltage V_(gd) which exceeds approximately 20 V cannot be applied to the semiconductor device, and it is difficult to obtain a high output from the semiconductor device.

(Semiconductor Device)

Next, a description will be given of a semiconductor device in a first embodiment, by referring to FIG. 3. FIG. 3 is a structure diagram illustrating the semiconductor device in the first embodiment. The semiconductor device illustrated in FIG. 3 includes a buffer layer (not illustrated), an i-GaN electron transit layer 21, an AlN intermediate layer 22, and an InAlN electron supply layer 23 that are stacked on a substrate 10. A source electrode 32 and a drain electrode 33 are provided on the electron supply layer 23, and a gate electrode 31 is provided on an insulator layer 50 that is provided on the electron supply layer 23 to form a gate insulator layer. A protection layer 40 made of SiN or the like is formed on an exposed region of the electron supply layer 23. In this embodiment, the substrate 10 is made of semi-insulating SiC, and a 2DEG 21 a is generated in the electron transit layer 21 at a vicinity of an interface between the electron transit layer 21 and the intermediate layer 22. In this embodiment, the electron transit layer 21 may also be referred to as “a first semiconductor layer”, the electron supply layer 23 may also be referred to as “a second semiconductor layer”, and the intermediate layer 22 may also be referred to as “a third semiconductor layer”.

The insulator layer 50 of the semiconductor device in this embodiment is formed by oxidizing the electron supply layer 23. By forming the insulator layer 50 under the gate electrode 31, it is possible to improve insulation directly under the gate electrode 31, reduce the gate leak current, and obtain a high output from the semiconductor device.

In this embodiment, the insulator layer 50 is formed by oxidizing a part of the electron supply layer 23. However, because the electron supply layer 23 is formed by InAlN, the insulator layer 50 includes In₂O₃ and Al₂O₃ that are formed when InAlN is oxidized. In₂O₃ has a narrow band gap and a poor capability of preventing the gate leak current. In addition, In₂O₃ is unstable, and inconsistencies or the like are likely to occur in properties thereof. Hence, In₂O₃ is not a preferable material for forming the insulator layer 50. On the other hand, Al₂O₃ has a wide band gap and is stable. Thus, Al₂O₃ is a preferable material for forming the insulator layer 50. Accordingly, in order to obtain a high insulation by the insulator layer 50, an Al₂O₃-content of the insulator layer 50 is preferably higher than an In₂O₃-content of the insulator layer 50. The insulator layer 50 of the semiconductor device in this embodiment includes more Al₂O₃ than In₂O₃. As a result, the gate leak current can be reduced, and generation of current collapse can be suppressed.

In this embodiment, the gate electrode 31 is a so-called T-shaped gate electrode having a T-shape. However, the gate electrode 31 is not limited to the T-shaped gate electrode, and other gate electrodes, such as an overhanging type gate electrode and a rectangular gate electrode, may be used for the gate electrode 31. In this embodiment, the insulator has a thickness of 1 nm or greater, preferably 5 nm or greater, and more preferably 1 nm or greater and 3 nm or less.

[Oxidation of InAlN]

Methods of oxidizing the nitride semiconductor, such as InAlN, or the like, include H₂O oxidation using water vapor (or water vapor-assisted H2 oxidation), O plasma oxidation, a thermal oxidation using oxygen, or the like. In the thermal oxidation using oxygen, the temperature at the time of the oxidation is approximately 600° C. and relatively high, which is not preferable since the high temperature may damage the semiconductor device that is manufactured. In the H₂O oxidation using water vapor and the O plasma oxidation, the temperature at the time of the oxidation is approximately 300° C. and relatively low, which enables the semiconductor device to be manufactured without being damaged.

Next, a description will be given of analysis results of the insulator layer analyzed by XPS (X-ray Photoelectron Spectroscopy), by referring to FIGS. 4A and 4B. FIG. 4A illustrates the analysis results for a case in which the insulator layer analyzed by the XPS is an oxide obtained by subjecting InAlN to the H₂O oxidation using water vapor, and FIG. 4B illustrates the analysis results for a case in which the insulator layer analyzed by the XPS is an oxide obtained by subjecting InAlN to the O plasma oxidation. The H₂O oxidation of InAlN using water vapor and the O plasma oxidation of InAlN are both performed at 300° C.

As illustrated in FIG. 4A, in the oxide obtained by subjecting InAlN to the H₂O oxidation using water vapor, more Al—O is observed than In—O. On the other hand, as illustrated in FIG. 4B, in the oxide obtained by subjecting InAlN to the O plasma oxidation, more In—O is observed than Al—O. Accordingly, the oxide obtained by subjecting InAlN to the H₂O oxidation using water vapor includes more Al₂O₂ than the oxide obtained by subjecting InAlN to the O plasma oxidation. For this reason, the insulation can be improved by forming the insulator layer 50 by the H₂O oxidation using water vapor, according to which more Al—O is observed than In—O, when compared to the O plasma oxidation.

FIG. 5 is a diagram illustrating a correlation between the oxidation methods and an Al/In ratio when forming the insulator layer. FIG. 5 illustrates the Al/In ratio of the Al-content with respect to the In-content in the InAlN before oxidation, the oxide obtained by oxidizing InAlN by the H₂O oxidation using water vapor, and the oxide obtained by oxidizing InAlN by the O plasma oxidation. A composition of InAlN is In₁₇Al₈₃N. As illustrated in FIG. 5, the Al/In ratio becomes high when InAlN is subjected to the H₂O oxidation using water vapor and also when InAlN is subjected to the O plasma oxidation. In addition, the Al/In ratio is higher for the case in which InAlN is subjected to the H₂O oxidation using water vapor, when compared to the case in which InAlN is subjected to the O plasma oxidation. In other words, the In-content in the oxide obtained by oxidizing InAlN by the H₂O oxidation using water vapor is reduced compared to the oxide obtained by oxidizing InAlN by the O plasma oxidation.

(H₂O Oxidation Using Water Vapor)

Next, a description will be given on why the Al/In ratio becomes high in the case in which InAlN is subjected to the H₂O oxidation using water vapor, when compared to the case in which InAlN is subjected to the O plasma oxidation.

In the following, a formula (1) represents a chemical reaction formula for a case in which Al is oxidized by the H₂O oxidation using water vapor. Formulas (2) and (3) represent chemical reaction formulas during reaction processes of the chemical reaction formula (1). In addition, a formula (4) represents a chemical reaction formula for a case in which In is oxidized by the H₂O oxidation using water vapor. Formulas (5) and (6) represent chemical reaction formulas during reaction processes of the chemical reaction formula (4). 2Al+6H₂O→Al₂O₃+3H₂O+3H₂-845 kJ/mol  (1) 2Al+6H₂O→2Al(OH)₃+3H₂  (2) 2Al(OH)₃→Al₂O₃+3H₂O  (3) 2In+6H₂O→In₂O₃+3H₂O+3H₂-91 kJ/mol  (4) 2In+6H₂O→2In₂(OH)₃+3H₂  (5) 2In(OH)₃→In₂O₃+H₂O  (6)

As represented by the formula (1), Al₂O₃ is generated by oxidizing Al included in InAlN by the H₂O oxidation using water vapor. This reaction first generates Al(OH)₃ by oxidizing Al included in InAlN by the H₂O oxidation using water vapor, as represented by the formula (2), and thereafter generates Al₂O₃ from Al(OH)₃ via a dehydration reaction, as represented by the formula (3).

In addition, as represented by the formula (4), In₂O₃ is generated by oxidizing In included in InAlN by the H₂O oxidation using water vapor. This reaction first generates In(OH)₃ by oxidizing In included in InAlN by the H₂O oxidation using water vapor, as represented by the formula (5), and thereafter generates In₂O₃ from In via a dehydration reaction, as represented by the formula (6).

Accordingly, in the case in which InAlN is subjected to the H₂O oxidation using water vapor, hydroxide is generated, and thereafter, the oxide is generated from the hydroxide via the dehydration reaction.

Al(OH)₃ generated in the formula (2) is a solid, however, In(OH)₃ generated in the formula (5) has an unstable solid phase and easily vaporizes. Accordingly, in the process of oxidizing InAlN by the H₂O oxidation using water vapor, a part of In vaporizes, to thereby reduce the In-content of the oxide that is obtained by oxidizing InAlN by the H₂O oxidation using water vapor. As a result, the Al/In ratio becomes high.

On the other hand, in the case in which InAlN is oxidized by the O plasma oxidation, no In(OH)₃ is generated, and thus, a large amount of In remains within the oxide without being vaporized or the like. It may be regarded, for this reason, that the Al/In ratio of the oxide that is obtained by oxidizing InAlN by the H₂O oxidation using water vapor is high compared to that of the oxide that is obtained by oxidizing InAlN by the O plasma oxidation.

In this embodiment, the substrate or the like is heated when oxidizing InAlN by the H₂O oxidation using water vapor. In order to smoothly perform the oxidation without damaging the nitride semiconductor layer by this heating, the heating is preferably performed to a temperature of 150° C. or higher and 550° C. or lower, and more preferably performed to a temperature of 200° C. or higher and 400° C. or lower.

(Properties of Semiconductor Device)

Next, a description will be given of the properties of the semiconductor device in this embodiment, by referring to FIGS. 6 and 7. FIGS. 6 and 7 illustrate properties of the current value of the gate leak current that flows in a case in which the potential of the source electrode or the drain electrode is 0 V and the potential of the gate electrode is varied. FIG. 6 is a diagram illustrating a correlation between the gate voltage and the gate leak current of the semiconductor device in this embodiment. FIG. 7 is a diagram illustrating a correlation between the gate voltage and the gate leak current of the semiconductor device illustrated in FIG. 1. As illustrated in FIGS. 6 and 7, the gate leak current of the semiconductor device in this embodiment can be reduced by 3 or more digits compared to that of the semiconductor device illustrated in FIG. 1. In other words, the gate leak current of the semiconductor device can be reduced compared to the semiconductor device illustrated in FIG. 1. The gate leak current of the semiconductor device in this embodiment can be reduced, because the insulator layer 50 is provided directly under the gate electrode 31.

(Method of Manufacturing Semiconductor Device)

Next, a description will be given of the method of manufacturing the semiconductor device in the first embodiment, by referring to FIGS. 8A through 13C. FIGS. 8A through 13C are diagrams for explaining processes of the method of manufacturing the semiconductor device in this embodiment.

First, as illustrated in FIG. 8A, the buffer layer (not illustrated), the electron transit layer 21, the intermediate layer 22, and the electron supply layer 23 are expitaxially grown on the substrate 10 by MOVPE (Metal Organic Vapor Phase Epitaxy) and successively stacked. In this embodiment, each of the buffer layer (not illustrated), the electron transit layer 21, the intermediate layer 22, and the electron supply layer 23 may also be referred to as “a nitride semiconductor layer”. The electron transit layer 21 is formed by i-GaN to a thickness of approximately 3 μm, the intermediate layer 22 is formed by i-AlN to a thickness of approximately 1 nm, and the electron supply layer 23 is formed by i-In_(0.17)Al_(0.83)N to a thickness of approximately 10 nm. Hence, the 2DEG 21 a is generated in the electron transit layer 21 at the vicinity of the interface between the electron transit layer 21 and the intermediate layer 22. The substrate 10 is formed by semi-insulating SiC, and the buffer layer (not illustrated) is formed by GaN, AlGaN, or the like.

Next, as illustrated in FIG. 8B, an isolation region 61 is formed in the nitride semiconductor layers that are formed on the substrate 10. More particularly, a photoresist is coated on the electron supply layer 23, and a resist pattern (not illustrated), having an opening in a region where the isolation region 61 is to be formed, is formed by exposing the photoresist by an exposure apparatus and developing the exposed photoresist. Thereafter, Ar ions or the like are injected into the nitride semiconductor layers within the opening of the resist pattern, in order to form the isolation region 61. When forming the isolation region 61, the Ar ions or the like may be injected into a part of the substrate 10. The resist pattern is thereafter removed using an organic solvent or the like.

Next, as illustrated in FIG. 8C, a resist pattern 71, having openings 71 a and 71 b in regions where the source electrode 32 and the drain electrode 33 are to be formed, is formed on the electron supply layer 23, and then, a part at the surface of the electron supply layer 23 is removed within the openings 71 a and 71 b. More particularly, a photoresist is coated on the electron supply layer 23, and the resist pattern 71, having the openings 71 a and 71 b in the regions where the source electrode 32 and the drain electrode 33 are to be formed, is formed by exposing the photoresist by the exposure apparatus and developing the exposed photoresist. Thereafter, the part at the surface of the electron supply layer 23, exposed within the openings 71 a and 71 b of the resist pattern 71 and not covered by the photoresist of the resist pattern 71, is removed by RIE (Reactive Ion Etching) or the like. The RIE may use, as an etching gas, a gas including a chlorine component.

Next, as illustrated in FIG. 9A, the resist pattern 71 is removed using an organic solvent or the like, to form a resist pattern 72 having openings 72 a and 72 b in the regions where the source electrode 32 and the drain electrode 33 are to be formed. More particularly, after removing the resist pattern 71 using the organic solvent or the like, a photoresist is again coated on the electron supply layer 23, and the photoresist is exposed by the exposure apparatus and developed. Hence, the resist pattern 72, having the openings 72 a and 72 b in the regions where the source electrode 32 and the drain electrode 33 are to be formed, is formed. As illustrated in FIG. 9A, the resist pattern 72 is formed by stacking 2 resist layers.

Next, as illustrated in FIG. 9B, a metal multilayer 81 made of Ti/Al is deposited on the surface of the stacked structure formed with the resist pattern 72, by vacuum deposition. More particularly, a Ti layer is deposited on the surface of the stacked structure formed with the resist pattern 72, by vacuum deposition, and an Al layer is thereafter deposited on the Ti layer by vacuum deposition. In this embodiment, the Ti layer is deposited to a thickness of approximately 20 nm, and the Al layer is deposited to a thickness of approximately 200 nm.

Next, as illustrated in FIG. 9C, the metal multilayer 81 formed on the resist pattern 72 is removed together with the resist pattern 72 by lift-off, by dipping the stacked structure in an organic solvent or the like. As a result, the source electrode 32 and the drain electrode 33 are formed by the metal multilayer 81 remaining within the regions where the openings 72 a and 72 b of the resist pattern 72 were formed. Thereafter, a thermal process is performed at a temperature of 550° C., in order to establish an ohmic contact between the electron supply layer 23 and each of the source electrode 32 and the drain electrode 33.

Next, as illustrated in FIG. 10A, an insulator layer (hereinafter also referred to as “a hard mask insulator layer”) 73 for forming a hard mask which will be described later is formed on the electron supply layer 23, the source electrode 32, and the drain electrode 33. More particularly, an SiN layer having a thickness of approximately 20 nm is deposited on the electron supply layer 23, the source electrode 32, and the drain electrode 33 by plasma CVD (Chemical Vapor Deposition), to form the hard mask insulator layer 73. When forming the hard mask insulator layer 73 by the plasma CVD, silane, ammonia, or the like may be used for a source gas, for example. A refractive index of the hard mask insulator layer 73 at a wavelength of 633 nm is 2.0, and the hard mask insulator layer 73 is a stoichiometric layer.

Next, as illustrated in FIG. 10B, a resist pattern 74 having an opening 74 a is formed on the hard mask insulator layer 73. More particularly, a photoresist is coated on the hard mask insulator layer 73, and the resist pattern 74 having the opening 74 a is formed by exposing the photoresist by the exposure apparatus and developing the exposed photoresist. The opening 74 a of the resist pattern 74 has a width of approximately 0.5 μm, and has a size identical to the size of the insulator layer 50 that is formed by oxidizing the electron supply layer 23, as will be described later in the specification.

Next, as illustrated in FIG. 10C, an opening 73 a is formed by removing the hard mask insulator layer 73 exposed within the opening 74 a of the resist pattern 74 by RIE or the like. As a result, a hard mask 73 b having the opening 73 a is formed by the remaining hard mask insulator layer 73. The resist pattern 74 is thereafter removed using an organic solvent or the like.

Next, as illustrated in FIG. 11A, the insulator layer 50 is formed by oxidizing the surface of the electron supply layer 23 exposed within the opening 73 a of the hard mask 73 b, by the H₂O oxidation using water vapor. More particularly, an ALD (Atomic Layer Deposition) apparatus is used to form the insulator layer 50 by setting the substrate temperature to 300° C., supplying water vapor (H₂O) which becomes an oxidation source, and oxidizing the surface of the electron supply layer 23 exposed within the opening 73 a of the hard mask 73 b. The insulator layer 50 that is formed in this manner has a thickness of approximately 3 nm, and a width D1 of approximately 0.5 μm.

Next, as illustrated in FIG. 11B, a resist pattern 75 is formed on the insulator layer 50. More particularly, a photoresist is formed on the electron supply layer 23, the insulator layer 50, the source electrode 32, and the drain electrode 33, and the resist pattern 75 is formed by exposing the photoresist by the exposure apparatus and developing the exposure photoresist.

Next, as illustrated in FIG. 11C, the hard mask 73 b in regions where no resist pattern 75 is formed is removed using a buffered hydrogen fluoride or the like. Thereafter, the resist pattern 75 is removed using an organic solvent or the like.

Next, as illustrated in FIG. 12A, a protection layer 40, which becomes a passivation layer, is formed on the electron supply layer 23, the insulator layer 50, the source electrode 32, and the drain electrode 33. More particularly, an SiN layer having a thickness of approximately 50 nm is deposited on the electron supply layer 23, the insulator layer 50, the source electrode 32, and the drain electrode 33 by plasma CVD, to form the protection layer 40 that becomes the passivation layer. When forming the protection layer 40 by the plasma CVD, silane, ammonia, or the like may be used for a source gas, for example. A refractive index of the protection layer 40 at the wavelength of 633 nm is 2.0, and the protection layer 40 is a stoichiometric layer. In this embodiment, after performing the process illustrated in FIG. 11A, the process illustrated in FIG. 12A may be performed by omitting the processes illustrated in FIGS. 11B and 11C. In this case, the SiN protection layer 40 is formed on the SiN hard mask 73 b, the insulator layer 50, the source electrode 32, and the drain electrode 33.

Next, as illustrated in FIG. 12B, a resist pattern 76 having an opening 76 a is formed on the protection layer 40. More particularly, an electron beam resist is coated on the protection layer 40, and the resist pattern 76 having the opening 76 a is formed by exposing the electron beam resist by an electron beam lithography apparatus and developing the exposed electron beam resist. The opening 76 a of the resist pattern 76 that is formed in this manner corresponds to a region where the gate electrode 31 is to be formed, as will be described later, and has a width of approximately 0.15 μm.

Next, as illustrated in FIG. 12C, the protection layer 40 within the opening 76 a of the resist pattern 76 is removed, in order to form an opening 40 a in the region where the gate electrode 31 is to be formed. More particularly, the protection layer 40 within the opening 76 a of the resist pattern 76 is removed by a dry etching using SF₆ as an etching gas, or the like, in order to form the opening 40 a in the protection layer 40. This opening 40 a is formed to correspond to the region where the gate electrode 31 is to be formed, and has a width D2 of approximately 0.15 μm.

Next, as illustrated in FIG. 13A, after removing the resist pattern 76 by an organic solvent or the like, a resist pattern 77 for forming the gate electrode 31 is formed on the protection layer 40. This resist pattern 77 is formed by 3 electron beam resist layers that are stacked, and has an opening 77 a in the region where the gate electrode 31 is to be formed. More particularly, the 3 electron beam resist layers are successively formed on the protection layer 40 by repeatedly coating the same, for example. In addition, exposing the electron beam resist by the electron beam lithography apparatus and developing the exposed electron beam resist are repeated for each of the 3 electron beam resist layers, in order to form the opening 77 a in the 3 electron beam resist layers. As a result, the opening 77 a is formed in the resist pattern 77. The opening 77 a of the resist pattern 77 is formed to have a width of 0.8 μm, 1.3 μm, and 0.2 μm for the top, middle, and bottom electron beam resist layers forming the 3 electron beam resist layers.

Next, as illustrated in FIG. 13B, a metal multilayer 82 made of Ni/Au is deposited on the surface of the stacked structure formed with the resist pattern 77, by vacuum deposition. More particularly, an Ni layer is deposited on the surface of the stacked structure formed with the resist pattern 77, by vacuum deposition, and an Au layer is thereafter deposited on the Ni layer by vacuum deposition. In this embodiment, the Ni layer is deposited to a thickness of approximately 10 nm, and the Au layer is deposited to a thickness of approximately 300 nm.

Next, as illustrated in FIG. 13C, the metal multilayer 82 formed on the resist pattern 77 is removed together with the resist pattern 77 by lift-off, by dipping the stacked structure in an organic solvent or the like. As a result, the gate electrode 31 is formed by the metal multilayer 82 remaining within the region where the opening 77 a of the resist pattern 77 was formed.

The semiconductor device in this embodiment is manufactured by the processes described above.

Second Embodiment

(Semiconductor Device)

Next, a description will be given of the semiconductor device in a second embodiment, by referring to FIG. 14. FIG. 14 is a structure diagram illustrating the semiconductor device in the second embodiment. The semiconductor device illustrated in FIG. 14 includes a buffer layer (not illustrated), an i-GaN electron transit layer 21, an AlN intermediate layer 22, and an InAlN electron supply layer 23 that are stacked on a substrate 10. A source electrode 32 and a drain electrode 33 are provided on the electron supply layer 23, and a gate electrode 31 is provided on an insulator layer 150 that is provided on the electron supply layer 23 to form a gate insulator layer, at a position closer to the source electrode 32 than the drain electrode 33. A protection layer 40 made of SiN or the like is formed on the exposed region of the electron supply layer 23. In this embodiment, the substrate 10 is made of semi-insulating SiC, and a 2DEG 21 a is generated in the electron transit layer 21 at the vicinity of the interface between the electron transit layer 21 and the intermediate layer 22.

The insulator layer 150 of the semiconductor device in this embodiment is formed by oxidizing the electron supply layer 23. The insulator layer 150 is not only formed in a region directly under the gate electrode 31, but is also formed in a region from the region directly under the gate electrode 31 to an intermediate part between the gate electrode 31 and the drain electrode 33. Hence, it is possible to further improve the insulation provided by the insulator layer 150, and obtain a high output from the semiconductor device.

Because the insulator layer 150 is formed by oxidizing the electron supply layer 23, thickness of the electron supply layer 23 in the region where the insulator layer 150 is formed is smaller than the thickness of the electron supply layer 23 in the region where no insulator layer 150 is formed. When the thickness of the electron supply layer 23 becomes smaller in the region where the insulator layer 150 is formed, the 2DEG 21 a directly under this region decreases. In the case of the semiconductor device in this embodiment having the electron transit layer 21 formed by i-GaN and the electron supply layer 23 formed by InAlN, the 2DEG 21 a having a high concentration is generated in the electron transit layer 21 at the vicinity of the interface between the electron transit layer 21 and the intermediate layer 22. In the case in which the 2DEG 21 a in the electron transit layer 21 has the high concentration, it is possible to increase the drain current, however, a gate breakdown voltage and an off-state breakdown voltage decrease.

Accordingly, in this embodiment, the insulator layer 150 is formed in the region from the gate electrode 31 to the intermediate part between the gate electrode 31 and the drain electrode 33, in order to reduce the thickness of the electron supply layer 23 in the region where the insulator layer 150 is formed, and decrease the density of the 2DEG 21 a that is generated. By decreasing the density of the 2DEG 21 a directly under the region from the gate electrode 31 to the intermediate part between the gate electrode 31 and the drain electrode 33, it becomes possible to improve the gate breakdown voltage and the off-state breakdown voltage, and obtain the high output from the semiconductor device. In addition, as illustrated in FIG. 15, a depletion layer region 151 can be extended to a desired region, in order to improve a high-frequency response of the semiconductor device.

FIG. 15 is a diagram for explaining the semiconductor device in the second embodiment. In FIG. 15, a two-dot chain line indicates a spread of the depletion layer region 151 when a voltage is applied to the gate electrode 31. As illustrated in FIG. 15, the depletion layer region 151 can easily be extended between the gate electrode 31 and the drain electrode 33, to the region in which the 2DEG 21 a is generated.

In this embodiment, a thickness T1 of the electron supply layer 23 in the region where no insulator layer 150 is formed is 10 nm, and in a case in which the thickness of the insulator layer 150 is 3 nm, a thickness T2 of the electron supply layer 23 in the region where the insulator layer 150 is formed is 7 nm.

(Properties of Semiconductor Device)

Next, a description will be given of properties of the semiconductor device in the first embodiment and the semiconductor device in the second embodiment, by referring to FIG. 16. FIG. 16 is a diagram for explaining the properties of the semiconductor devices in the first embodiment and the second embodiment. As illustrated in FIG. 16, a gate-source capacitance C_(gs) is 500 (fF/mm) and the same for the semiconductor device in the first embodiment and the semiconductor device in the second embodiment. In addition, a drain-source capacitance C_(ds) is 150 (fF/mm) and the same for the semiconductor device in the first embodiment and the semiconductor device in the second embodiment.

On the other hand, a gate-drain capacitance C_(gd) is 130 (fF/mm) for the semiconductor device in the first embodiment, but is 110 (fF/mm) for the semiconductor device in the second embodiment. In other words, the gate-drain capacitance C_(gd) of the semiconductor device in the second embodiment is lower than that of the semiconductor device in the first embodiment.

In addition, a maximum transmission frequency f_(max) is 240 (GHz) for the semiconductor device in the first embodiment, but is 270 (GHz) for the semiconductor device in the second embodiment. In other words, the maximum transmission frequency f_(max) of the semiconductor device in the second embodiment is higher than that of the semiconductor device in the first embodiment. Accordingly, the frequency response of the semiconductor device in the second embodiment can further be improved compared to that of the semiconductor device in the first embodiment.

(Method of Manufacturing Semiconductor Device)

Next, a description will be given of the method of manufacturing the semiconductor device in the second embodiment, by referring to FIGS. 17A through 22C. FIGS. 17A through 22C are diagrams for explaining processes of the method of manufacturing the semiconductor device in this embodiment.

First, as illustrated in FIG. 17A, the buffer layer (not illustrated), the electron transit layer 21, the intermediate layer 22, and the electron supply layer 23 are expitaxially grown on the substrate 10 by MOVPE and successively stacked. In this embodiment, each of the buffer layer (not illustrated), the electron transit layer 21, the intermediate layer 22, and the electron supply layer 23 may also be referred to as “a nitride semiconductor layer”. The electron transit layer 21 is formed by i-GaN to a thickness of approximately 3 μm, the intermediate layer 22 is formed by i-AlN to a thickness of approximately 1 nm, and the electron supply layer 23 is formed by i-In_(0.17)Al_(0.83)N to a thickness of approximately 10 nm. Hence, the 2DEG 21 a is generated in the electron transit layer 21 at the vicinity of the interface between the electron transit layer 21 and the intermediate layer 22. The substrate 10 is formed by semi-insulating SiC, and the buffer layer (not illustrated) is formed by GaN, AlGaN, or the like.

Next, as illustrated in FIG. 17B, an isolation region 61 is formed in the nitride semiconductor layers that are formed on the substrate 10. More particularly, a photoresist is coated on the electron supply layer 23, and a resist pattern (not illustrated), having an opening in a region where the isolation region 61 is to be formed, is formed by exposing the photoresist by the exposure apparatus and developing the exposed photoresist. Thereafter, Ar ions or the like are injected into the nitride semiconductor layers within the opening of the resist pattern, in order to form the isolation region 61. When forming the isolation region 61, the Ar ions or the like may be injected into a part of the substrate 10. The resist pattern is thereafter removed using an organic solvent or the like.

Next, as illustrated in FIG. 17C, a resist pattern 71, having openings 71 a and 71 b in regions where the source electrode 32 and the drain electrode 33 are to be formed, is formed on the electron supply layer 23, and then, a part at the surface of the electron supply layer 23 is removed within the openings 71 a and 71 b. More particularly, a photoresist is coated on the electron supply layer 23, and the resist pattern 71, having the openings 71 a and 71 b in the regions where the source electrode 32 and the drain electrode 33 are to be formed, is formed by exposing the photoresist by the exposure apparatus and developing the exposed photoresist. Thereafter, the part at the surface of the electron supply layer 23, exposed within the openings 71 a and 71 b of the resist pattern 71 and not covered by the photoresist of the resist pattern 71, is removed by RIE or the like. The RIE may use, as an etching gas, a gas including a chlorine component.

Next, as illustrated in FIG. 18A, the resist pattern 71 is removed using an organic solvent or the like, to form a resist pattern 72 having openings 72 a and 72 b in the regions where the source electrode 32 and the drain electrode 33 are to be formed. More particularly, after removing the resist pattern 71 using the organic solvent or the like, a photoresist is again coated on the electron supply layer 23, and the photoresist is exposed by the exposure apparatus and developed. Hence, the resist pattern 72, having the openings 72 a and 72 b in the regions where the source electrode 32 and the drain electrode 33 are to be formed, is formed. As illustrated in FIG. 18A, the resist pattern 72 is formed by stacking 2 resist layers.

Next, as illustrated in FIG. 18B, a metal multilayer 81 made of Ti/Al is deposited on the surface of the stacked structure formed with the resist pattern 72, by vacuum deposition. More particularly, a Ti layer is deposited on the surface of the stacked structure formed with the resist pattern 72, by vacuum deposition, and an Al layer is thereafter deposited on the Ti layer by vacuum deposition. In this embodiment, the Ti layer is deposited to a thickness of approximately 20 nm, and the Al layer is deposited to a thickness of approximately 200 nm.

Next, as illustrated in FIG. 18C, the metal multilayer 81 formed on the resist pattern 72 is removed together with the resist pattern 72 by lift-off, by dipping the stacked structure in an organic solvent or the like. As a result, the source electrode 32 and the drain electrode 33 are formed by the metal multilayer 81 remaining within the regions where the openings 72 a and 72 b of the resist pattern 72 were formed. Thereafter, a thermal process is performed at a temperature of 550° C., in order to establish an ohmic contact between the electron supply layer 23 and each of the source electrode 32 and the drain electrode 33.

Next, as illustrated in FIG. 19A, an insulator layer (hereinafter also referred to as “a hard mask insulator layer”) 173 for forming a hard mask which will be described later is formed on the electron supply layer 23, the source electrode 32, and the drain electrode 33. More particularly, an SiN layer having a thickness of approximately 20 nm is deposited on the electron supply layer 23, the source electrode 32, and the drain electrode 33 by plasma CVD, to form the hard mask insulator layer 173. When forming the hard mask insulator layer 173 by the plasma CVD, silane, ammonia, or the like may be used for a source gas, for example. A refractive index of the hard mask insulator layer 173 at the wavelength of 633 nm is 2.0, and the hard mask insulator layer 173 is a stoichiometric layer.

Next, as illustrated in FIG. 19B, a resist pattern 174 having an opening 174 a is formed on the hard mask insulator layer 173. More particularly, a photoresist is coated on the hard mask insulator layer 173, and the resist pattern 174 having the opening 174 a is formed by exposing the photoresist by the exposure apparatus and developing the exposed photoresist. The opening 174 a of the resist pattern 174 has a width of approximately 1.0 μm, and has a size identical to the size of the insulator layer 150 that is formed by oxidizing the electron supply layer 23, as will be described later in the specification. In this embodiment, the insulator layer 150 partially extends from a region directly under the gate electrode 31 towards the drain electrode 33.

Next, as illustrated in FIG. 19C, an opening 173 a is formed by removing the hard mask insulator layer 173 exposed within the opening 174 a of the resist pattern 174 by RIE or the like. As a result, a hard mask 173 b having the opening 173 a is formed by the remaining hard mask insulator layer 173. The resist pattern 174 is thereafter removed using an organic solvent or the like.

Next, as illustrated in FIG. 20A, the insulator layer 150 is formed by oxidizing the surface of the electron supply layer 23 exposed within the opening 173 a of the hard mask 173 b, by the H₂O oxidation using water vapor. More particularly, the ALD apparatus is used to form the insulator layer 150 by setting the substrate temperature to 300° C., supplying water vapor (H₂O) which becomes an oxidation source, and oxidizing the surface of the electron supply layer 23 exposed within the opening 173 a of the hard mask 173 b. The insulator layer 150 that is formed in this manner has a thickness of approximately 3 nm, and a width D3 of approximately 1.0 μm so as to partially extend from the region directly under the gate electrode 31 towards the drain electrode 33.

Next, as illustrated in FIG. 20B, a resist pattern 175 is formed on the insulator layer 150. More particularly, a photoresist is formed on the electron supply layer 23, the insulator layer 150, the source electrode 32, and the drain electrode 33, and the resist pattern 175 is formed by exposing the photoresist by the exposure apparatus and developing the exposure photoresist.

Next, as illustrated in FIG. 20C, the hard mask 173 b in regions where no resist pattern 175 is formed is removed using a buffered hydrogen fluoride or the like. Thereafter, the resist pattern 175 is removed using an organic solvent or the like.

Next, as illustrated in FIG. 21A, a protection layer 40, which becomes a passivation layer, is formed on the electron supply layer 23, the insulator layer 150, the source electrode 32, and the drain electrode 33. More particularly, an SiN layer having a thickness of approximately 50 nm is deposited on the electron supply layer 23, the insulator layer 150, the source electrode 32, and the drain electrode 33 by plasma CVD, to form the protection layer 40 that becomes the passivation layer. When forming the protection layer 40 by the plasma CVD, silane, ammonia, or the like may be used for a source gas, for example. A refractive index of the protection layer 40 at the wavelength of 633 nm is 2.0, and the protection layer 40 is a stoichiometric layer. In this embodiment, after performing the process illustrated in FIG. 20A, the process illustrated in FIG. 21A may be performed by omitting the processes illustrated in FIGS. 20B and 20C. In this case, the SiN protection layer 40 is formed on the SiN hard mask 173 b, the insulator layer 150, the source electrode 32, and the drain electrode 33.

Next, as illustrated in FIG. 21B, a resist pattern 76 having an opening 76 a is formed on the protection layer 40. More particularly, an electron beam resist is coated on the protection layer 40, and the resist pattern 76 having the opening 76 a is formed by exposing the electron beam resist by the electron beam lithography apparatus and developing the exposed electron beam resist. The opening 76 a of the resist pattern 76 that is formed in this manner corresponds to a region where the gate electrode 31 is to be formed, as will be described later, and has a width of approximately 0.15 μm. The opening 76 a of the resist pattern 76 is formed so that, when the gate electrode 31 is formed, the insulator layer 150 partially extends from the region directly under the gate electrode 31 towards the drain electrode 33.

Next, as illustrated in FIG. 21C, the protection layer 40 within the opening 76 a of the resist pattern 76 is removed, in order to form an opening 40 a in the region where the gate electrode 31 is to be formed. More particularly, the protection layer 40 within the opening 76 a of the resist pattern 76 is removed by a dry etching using SF₆ as an etching gas, or the like, in order to form the opening 40 a in the protection layer 40. This opening 40 a is formed to correspond to the region where the gate electrode 31 is to be formed, and has a width D2 of approximately 0.15 μm.

Next, as illustrated in FIG. 22A, after removing the resist pattern 76 by an organic solvent or the like, a resist pattern 77 for forming the gate electrode 31 is formed on the protection layer 40. This resist pattern 77 is formed by 3 electron beam resist layers that are stacked, and has an opening 77 a in the region where the gate electrode 31 is to be formed. More particularly, the 3 electron beam resist layers are successively formed on the protection layer 40 by repeatedly coating the same, for example. In addition, exposing the electron beam resist by the electron beam lithography apparatus and developing the exposed electron beam resist are repeated for each of the 3 electron beam resist layers, in order to form the opening 77 a in the 3 electron beam resist layers. As a result, the opening 77 a is formed in the resist pattern 77. The opening 77 a of the resist pattern 77 is formed to have a width of 0.8 μm, 1.3 μm, and 0.2 μm for the top, middle, and bottom electron beam resist layers forming the 3 electron beam resist layers.

Next, as illustrated in FIG. 22B, a metal multilayer 82 made of Ni/Au is deposited on the surface of the stacked structure formed with the resist pattern 77, by vacuum deposition. More particularly, an Ni layer is deposited on the surface of the stacked structure formed with the resist pattern 77, by vacuum deposition, and an Au layer is thereafter deposited on the Ni layer by vacuum deposition. In this embodiment, the Ni layer is deposited to a thickness of approximately 10 nm, and the Au layer is deposited to a thickness of approximately 300 nm.

Next, as illustrated in FIG. 22C, the metal multilayer 82 formed on the resist pattern 77 is removed together with the resist pattern 77 by lift-off, by dipping the stacked structure in an organic solvent or the like. As a result, the gate electrode 31 is formed by the metal multilayer 82 remaining within the region where the opening 77 a of the resist pattern 77 was formed.

The semiconductor device in this embodiment is manufactured by the processes described above.

Other features of the second embodiment are the same as those of the first embodiment.

Third Embodiment

Next, a description will be given of a third embodiment. In this embodiment, the present invention is applied to a semiconductor device, a power supply unit, and a high-output amplifier (or high-frequency amplifier).

(Semiconductor Device)

The semiconductor device in this embodiment includes a discrete package of the semiconductor device in the first embodiment or the second embodiment described above. A description will be given of the discrete package of the semiconductor device, by referring to FIG. 23. FIG. 23 is a diagram for explaining the semiconductor device in the third embodiment. Because FIG. 23 schematically illustrates the inside of the discrete package including the semiconductor device, the electrodes or the like are arranged at locations that may not match those of the semiconductor device in the first or second embodiment.

First, the semiconductor device in the first or second embodiment is cut by dicing or the like, in order to form a semiconductor chip 410 of a HEMT that uses a GaN-based semiconductor material. This semiconductor chip 410 is fixed on a lead frame 420 by a die-attaching agent 430 such as solder or the like. The semiconductor chip 410 corresponds to the semiconductor device in the first or second embodiment.

Next, a gate electrode 411 is connected to a gate lead 421 by a bonding wire 431, a source electrode 412 is connected to a source lead 422 by a bonding wire 432, and a drain electrode 413 is connected to a drain lead 423 by a bonding wire 433. The bonding wires 431, 432, and 433 are made of a metal material, such as Al or the like. In addition, in this embodiment, the gate electrode 411 is a type of gate electrode pad, and is connected to the gate electrode 31 of the semiconductor device in the first or second embodiment. Moreover, the source electrode 412 is a type of source electrode pad, and is connected to the source electrode 32 of the semiconductor device in the first or second embodiment. Furthermore, the drain electrode 413 is a type of drain electrode pad, and is connected to the drain electrode 33 of the semiconductor device in the first or second embodiment.

Next, a mold resin 440 encapsulates the semiconductor chip 410 or the like by resin encapsulation using transfer molding. As a result, the discrete package of the semiconductor device in the first or second embodiment, including the HEMT that uses the GaN-based semiconductor material, can be manufactured.

(PFC Circuit, Power Supply Unit & High-Output Amplifier)

Next, a description will be given of the PFC circuit, the power supply unit, and the high-output amplifier (or high-frequency amplifier) in this embodiment. The PFC circuit, the power supply unit, and the high-output amplifier (or high-frequency amplifier) in this embodiment respectively are the power supply unit and the high-output amplifier (or high-frequency amplifier) using the semiconductor device in the first or second embodiment.

(PFC Circuit)

Next, a description will be given of the PFC (Power Factor Correction) circuit in this embodiment. The PFC circuit in this embodiment includes the semiconductor device in the first or second embodiment.

A description of the PFC circuit in this embodiment will be described with reference to FIG. 24. FIG. 24 is a circuit diagram illustrating the PFC circuit in the third embodiment. A PFC circuit 450 illustrated in FIG. 24 includes a switching device (or transistor) 451, a diode 452, a choke coil 453, capacitors 454 and 455, a diode bridge 456, and an AC (Alternating Current) power supply (not illustrated). The HEMT of the semiconductor device in the first or second embodiment is used for the switching device 451.

In the PFC circuit 450, a drain electrode of the switching device 451, an anode terminal of the diode 452, and one terminal of the choke coil 453 are connected. In addition, a source electrode of the switching device 451, one terminal of the capacitor 454, and one terminal of the capacitor 455 are connected. The other terminal of the capacitor 454 and the other terminal of the choke coil 453 are connected. The other terminal of the capacitor 455 and a cathode terminal of the diode 452 are connected. The AC power supply is connected to the two terminals of the capacitor 454 via the diode bridge 456. In this PFC circuit 450, a DC (Direct Current) output is obtained from the two terminals of the capacitor 455.

(Power Supply Unit)

Next, a description will be given of the power supply unit in this embodiment. The power supply unit in this embodiment includes the semiconductor device in the first or second embodiment.

A description will be given of the power supply unit of this embodiment, by referring to FIG. 25. FIG. 25 is a circuit diagram illustrating the power supply unit in the third embodiment. The power supply unit illustrated in FIG. 25 includes the PFC circuit 450 of this embodiment described above.

The power supply unit in this embodiment includes a high-voltage primary side circuit 461, a low-voltage side secondary side circuit 462, and a transformer 463 arranged between the primary side circuit 461 and the secondary side circuit 462.

The primary side circuit 461 includes the PFC circuit 450 described above, an inverter circuit connected to the two terminals of the capacitor 455 in the PFC circuit 450, such as a full-bridge inverter circuit 460, for example. The full-bridge inverter circuit 460 includes a plurality of (4 in this example) switching devices 464 a, 464 b, 464 c, and 464 d. In addition, the secondary side circuit 462 includes a plurality of (3 in this example) switching devices 4651, 465 b, and 465 c. An AC power supply 457 is connected to the diode bridge 456.

In this embodiment, the HEMT of the semiconductor device in the first or second embodiment is used for the switching device 451 of the PFC circuit 450 in the primary side circuit 461. In addition, the HEMT of the semiconductor device in the first or second embodiment is used for the switching devices 464 a, 464 b, 464 c, and 464 d of the full-bridge inverter circuit 460 in the primary side circuit 461. On the other hand, an FET having a known MIS (Metal Insulator Semiconductor) structure using silicon, or the like, is used for the switching devices 465 a, 465 b, and 465 c of the secondary side circuit 462.

(High-Output Amplifier)

Next, a description will be given of the high-output amplifier (or high-frequency amplifier) in this embodiment. The high-output amplifier in this embodiment includes the semiconductor device in the first or second embodiment.

A description will be given of the high-output amplifier of this embodiment, by referring to FIG. 26. FIG. 26 is a structure diagram illustrating the high-output amplifier in the third embodiment. The high-output amplifier illustrated in FIG. 26 includes a digital predistortion circuit 471, mixers 472 a and 472 b, a power amplifier 473, and a directional coupler 474.

The digital predistortion circuit 471 compensates for a non-linear distortion in an input signal. The mixer 472 a mixes the input signal compensated of the non-linear distortion, and an AC signal. The power amplifier 473 amplifies the input signal mixed with the AC signal. In this embodiment, the HEMT of the semiconductor device in the first or second embodiment is used for the power amplifier 473. The directional coupler 474 monitors the input signal and an output signal, for example. In FIG. 26, the output signal may be mixed with the AC signal by the mixer 472 b, and the output signal mixed with the AC signal may be input to the digital predistortion circuit 471, by switching a switch, for example.

Fourth Embodiment

Next, a description will be given of embodiments provided with a gate insulator layer.

In the semiconductor device illustrated in FIG. 1 using InAlN for the electron supply layer 923, the gate leak current I_(g)leak may be reduced by a method using the MIS structure. FIG. 27 is a structure diagram illustrating the semiconductor device using this method and provided with a gate insulator layer. In FIG. 27, those parts that are the same as those corresponding parts in FIG. 1 are designated by the same reference numerals, and a description thereof will be omitted. In FIG. 27, a gate insulator layer 960 made of Al₂O₃ or the like is formed on the electron supply layer 923, and the gate electrode 931 is formed on the gate insulator layer 960. However, it is difficult to form a satisfactory gate insulator layer on InAlN, and in a case in which the gate insulator layer 960 is formed on the electron supply layer 923, a problem may be generated in that the current collapse may occur and cause the ON-resistance to become high. It may be regarded that the current collapse in the case in which the gate insulator layer 960 made of Al₂O₃ or the like is formed on the electron supply layer 923 made of InAlN is caused by an increase in current trap concentration at an interface between the InAlN and Al₂O₃.

(Semiconductor Device)

Next, a description will be given of the semiconductor device in a fourth embodiment, by referring to FIG. 28. FIG. 28 is a structure diagram illustrating the semiconductor device in the fourth embodiment. The semiconductor device illustrated in FIG. 28 includes a buffer layer (not illustrated), an i-GaN electron transit layer 21, an AlN intermediate layer 22, and an InAlN electron supply layer 23 that are stacked on a substrate 10. A source electrode 32 and a drain electrode 33 are provided on the electron supply layer 23. An oxide layer 50 is formed by oxidizing a material forming the electron supply layer 23, at a surface of the electron supply layer 23 immediately under a region where a gate electrode 31 is to be formed. An insulator layer 60 that becomes a gate insulator layer is provided on the electron supply layer 23 and the oxide layer 50. The insulator layer 60 is made of Al₂O₃ or the like. The gate electrode 31 is provided on the insulator layer 60 in a region where the oxide layer 50 is formed. The source electrode 32 and the drain electrode 33 may be provided on the electron transit layer 21, or on the intermediate layer 22.

In this embodiment, the substrate 10 is made of semi-insulating SiC, and a 2DEG 21 a is generated in the electron transit layer 21 at a vicinity of an interface between the electron transit layer 21 and the intermediate layer 22. In this embodiment, the electron transit layer 21 may also be referred to as “a first semiconductor layer”, the electron supply layer 23 may also be referred to as “a second semiconductor layer”, and the intermediate layer 22 may also be referred to as “a third semiconductor layer”.

The oxide layer 50 of the semiconductor device in this embodiment is formed by oxidizing the electron supply layer 23. By forming the oxide layer 50 under the gate electrode 31, it is possible to form a satisfactory insulator layer 60 on the oxide layer 50. As a result, it is possible to improve insulation directly under the gate electrode 31, reduce the gate leak current, and obtain a high output from the semiconductor device. In addition, in a case in which the insulator layer 60 is an oxide, the insulator layer 60 can be formed on the oxide layer 50 satisfactorily matching the material forming the insulator layer 60. In this case, the electrons are uneasily trapped and the electron trap concentration at an interface between the oxide layer 60 and the insulator layer 60, and for this reason, the generation of the current collapse can be suppressed.

In this embodiment, the oxide layer 50 is formed by oxidizing a part of the electron supply layer 23. However, because the electron supply layer 23 is formed by InAlN, the oxide layer 50 includes In₂O₃ and Al₂O₃ that are formed when the InAlN is oxidized. The In₂O₃ has a narrow band gap and a poor capability of preventing the gate leak current. In addition, the In₂O₃ is unstable, and inconsistencies or the like are likely to occur in properties thereof. Hence, In₂O₃ is not a preferable material for forming the oxide layer 50. On the other hand, Al₂O₃ has a wide band gap and is stable. Thus, Al₂O₃ is a preferable material for forming the oxide layer 50. Accordingly, in order to obtain a high insulation by the oxide layer 50, an Al₂O₃-content of the oxide layer 50 is preferably higher than an In₂O₃-content of the oxide layer 50. The oxide layer 50 of the semiconductor device in this embodiment includes more Al₂O₃ than In₂O₃. As a result, the gate leak current can be reduced, and generation of current collapse can be suppressed.

In this embodiment, the gate electrode 31 is a so-called T-shaped gate electrode having a T-shape. However, the gate electrode 31 is not limited to the T-shaped gate electrode, and other gate electrodes, such as an overhanging type gate electrode and a rectangular gate electrode, may be used for the gate electrode 31. In this embodiment, the oxide layer 50 has a thickness of 1 nm or greater, preferably 5 nm or greater, and more preferably 1 nm or greater and 3 nm or less. In addition, the insulator layer 60 may be formed by a material selected from oxides, nitrides, oxynitrides, or the like. However, because the oxide layer 50 includes a large Al oxide content, the insulator layer 60 is preferably made of Al₂O₃, AlN, AlON, or the like, and more preferably made of Al₂O₃ which is an Al oxide.

[Oxidation of InAlN]

Methods of oxidizing the nitride semiconductor, such as InAlN, or the like, include H₂O oxidation using water vapor (or water vapor-assisted H2 oxidation), O plasma oxidation, a thermal oxidation using oxygen, or the like. In the thermal oxidation using oxygen, the temperature at the time of the oxidation is approximately 600° C. and relatively high, which is not preferable since the high temperature may damage the semiconductor device that is manufactured. In the H₂O oxidation using water vapor and the O plasma oxidation, the temperature at the time of the oxidation is approximately 300° C. and relatively low, which enables the semiconductor device to be manufactured without being damaged.

Next, a description will be given of analysis results of the oxide layer analyzed by XPS (X-ray Photoelectron Spectroscopy), by referring to FIGS. 29A and 29B. FIG. 29A illustrates the analysis results for a case in which the oxide layer analyzed by the XPS is an oxide obtained by subjecting InAlN to the H₂O oxidation using water vapor, and FIG. 29B illustrates the analysis results for a case in which the oxide layer analyzed by the XPS is an oxide obtained by subjecting InAlN to the O plasma oxidation. The H₂O oxidation of InAlN using water vapor and the O plasma oxidation of InAlN are both performed at 300° C.

As illustrated in FIG. 29A, in the oxide obtained by subjecting InAlN to the H₂O oxidation using water vapor, more Al—O is observed than In—O. On the other hand, as illustrated in FIG. 29B, in the oxide obtained by subjecting InAlN to the O plasma oxidation, more In—O is observed than Al—O. Accordingly, the oxide obtained by subjecting InAlN to the H₂O oxidation using water vapor includes more Al₂O₃ than the oxide obtained by subjecting InAlN to the O plasma oxidation. For this reason, the insulation can be improved by forming the oxide layer 50 by the H₂O oxidation using water vapor, according to which more Al—O is observed than In—O, when compared to the O plasma oxidation.

FIG. 30 is a diagram illustrating a correlation between the oxidation methods and an Al/In ratio when forming the oxide layer. FIG. 5 illustrates the Al/In ratio of the Al-content with respect to the In-content in the InAlN before oxidation, the oxide obtained by oxidizing InAlN by the H₂O oxidation using water vapor, and the oxide obtained by oxidizing InAlN by the O plasma oxidation. A composition of InAlN is In₁₇Al₈₃N. As illustrated in FIG. 30, the Al/In ratio becomes high when InAlN is subjected to the H₂O oxidation using water vapor and also when InAlN is subjected to the O plasma oxidation. In addition, the Al/In ratio is higher for the case in which InAlN is subjected to the H₂O oxidation using water vapor, when compared to the case in which InAlN is subjected to the O plasma oxidation. In other words, the In-content in the oxide obtained by oxidizing InAlN by the H₂O oxidation using water vapor is reduced compared to the oxide obtained by oxidizing InAlN by the O plasma oxidation.

(H₂O Oxidation Using Water Vapor)

The reason why the Al/In ratio becomes high in the case in which InAlN is subjected to the H₂O oxidation using water vapor, when compared to the case in which InAlN is subjected to the O plasma oxidation, is as described above in conjunction with the first embodiment.

In this embodiment, the substrate or the like is heated when oxidizing InAlN by the H₂O oxidation using water vapor, similarly as in the case of the first embodiment described above.

In the semiconductor device in this embodiment, the oxide layer 50 is formed by oxidizing a part of the surface of the InAlN forming the electron supply layer 23, and the insulator layer 60 made of Al₂O₃ or the like is formed on the oxide layer 50. The method of oxidizing the part of the surface of the InAlN forming the electron supply layer 23 may be the H₂O oxidation using water vapor, or the O plasma oxidation. However, the H₂O oxidation using water vapor is more preferable in that the In-content can be reduced more and the composition ratio of Al can be made high.

As described above, when oxidizing the InAlN forming the electron supply layer 23 in order to form the oxide layer 50, the In-content is reduced and the Al-content is increased as the oxidation progresses. When forming the oxide layer 50, the oxidation gradually progresses from the surface of the InAlN. Hence, as illustrated in FIG. 31, at the surface of the oxide layer 50, the In-content is reduced more and more Al₂O₃ is formed than at a part of the oxide layer 50 deeper into the electron supply layer 23 from the surface of the oxide layer 50. FIG. 31 is a diagram illustrating a composition distribution of the oxide layer of the semiconductor device in the fourth embodiment.

In the case in which the insulator layer 60 is made of Al₂O₃, identical to the Al₂O₃ forming the oxide layer 50, the insulator 60 matches well to the oxide layer 50 when the insulator layer 60 is formed on the oxide layer 50. As a result, an interface trap is uneasily formed at the interface between the oxide layer 50 and the insulator layer 60. For this reason, the electrons are uneasily trapped between the oxide layer 50 and the insulator layer 60, and the generation of the current collapse can be suppressed.

(Properties of Semiconductor Device)

Next, a description will be given of the properties of the semiconductor device in this embodiment. FIGS. 32 and 33 illustrate measured results of the gate leak current that flows when a voltage is applied across the gate electrode and the drain electrode. In FIGS. 32 and 33, the abscissa indicates the applied voltage, and the ordinate indicates the gate leak current. FIG. 32 is a correlation diagram of the gate voltage and the leak current of the semiconductor device in the fourth embodiment illustrated in FIG. 28. FIG. 33 is a correlation diagram of the gate voltage and the leak current of the semiconductor device illustrated in FIG. 1.

According to the semiconductor device in this embodiment, no gate leak current flows even when the voltage of up to 16 V is applied across the gate electrode and the drain electrode, as illustrated in FIG. 32. On the other hand, according to the semiconductor device illustrated in FIG. 1, the gate leak current flows when the voltage is applied across the gate electrode and the drain electrode, as illustrated in FIG. 33. Hence, it is confirmed that the semiconductor device in this embodiment can suppress the generation of the gate leak current. The generation of the gate leak current is suppressed and the gate leak current can be reduced according to the semiconductor device in this embodiment, because the oxide layer 50 and the insulator layer 60 are formed immediately below the gate electrode 31.

Next, a description will be given of the current collapse of the semiconductor device in this embodiment, in comparison with the current collapse of the semiconductor device illustrated in FIG. 27. FIGS. 34 and 35 illustrate relationships between a drain voltage V_(ds) and a drain current I_(ds) when a gate voltage V_(gs) is varied, for a case in which the drain voltage V_(ds) is increased up to 10 V, and for a case in which the drain voltage V_(ds) is increased up to 20 V. FIG. 34 is a correlation diagram of the drain voltage and the drain current of the semiconductor device in the fourth embodiment illustrated in FIG. 28, and FIG. 35 is a correlation diagram of the drain voltage and the drain current of the semiconductor device illustrated in FIG. 27.

According to the semiconductor device having the structure illustrated in FIG. 27, the drain current I_(ds) decreases due to the current collapse in the case in which the drain voltage V_(ds) is increased up to 20 V, as illustrated in FIG. 35. On the other hand, according to the semiconductor device in this embodiment having the structure illustrated in FIG. 28, the drain current I_(ds) does not show a relative decrease even in the case in which the drain voltage V_(ds) is increased up to 20 V, and the generation of the current collapse is suppressed.

Therefore, according to the semiconductor device in this embodiment, it is possible to reduce the gate leak current. In addition, because the generation of the current collapse can be suppressed, it is possible to suppress the ON-resistance from becoming high.

(Method of Manufacturing Semiconductor Device)

Next, a description will be given of the method of manufacturing the semiconductor device in the fourth embodiment, by referring to FIGS. 36A through 41. FIGS. 36A through 41 are diagrams for explaining processes of the method of manufacturing the semiconductor device in this embodiment.

First, as illustrated in FIG. 36A, the buffer layer (not illustrated), the electron transit layer 21, the intermediate layer 22, and the electron supply layer 23 are expitaxially grown on the substrate 10 by MOVPE (Metal Organic Vapor Phase Epitaxy) and successively stacked. In this embodiment, each of the buffer layer (not illustrated), the electron transit layer 21, the intermediate layer 22, and the electron supply layer 23 may also be referred to as “a nitride semiconductor layer”. The electron transit layer 21 is formed by i-GaN to a thickness of approximately 3 μm, the intermediate layer 22 is formed by i-AlN to a thickness of approximately 1 nm, and the electron supply layer 23 is formed by i-In_(0.17)Al_(0.83)N to a thickness of approximately 10 nm. Hence, the 2DEG 21 a is generated in the electron transit layer 21 at the vicinity of the interface between the electron transit layer 21 and the intermediate layer 22. The substrate 10 is formed by semi-insulating SiC, and the buffer layer (not illustrated) is formed by GaN, AlGaN, or the like.

Next, as illustrated in FIG. 36B, an isolation region 61 is formed in the nitride semiconductor layers that are formed on the substrate 10. More particularly, a photoresist is coated on the electron supply layer 23, and a resist pattern (not illustrated), having an opening in a region where the isolation region 61 is to be formed, is formed by exposing the photoresist by an exposure apparatus and developing the exposed photoresist. Thereafter, Ar ions or the like are injected into the nitride semiconductor layers within the opening of the resist pattern, in order to form the isolation region 61. When forming the isolation region 61, the Ar ions or the like may be injected into a part of the substrate 10. The resist pattern is thereafter removed using an organic solvent or the like.

Next, as illustrated in FIG. 36C, a resist pattern 71, having openings 71 a and 71 b in regions where the source electrode 32 and the drain electrode 33 are to be formed, is formed on the electron supply layer 23, and then, a part at the surface of the electron supply layer 23 is removed within the openings 71 a and 71 b. More particularly, a photoresist is coated on the electron supply layer 23, and the resist pattern 71, having the openings 71 a and 71 b in the regions where the source electrode 32 and the drain electrode 33 are to be formed, is formed by exposing the photoresist by the exposure apparatus and developing the exposed photoresist. Thereafter, the part at the surface of the electron supply layer 23, exposed within the openings 71 a and 71 b of the resist pattern 71 and not covered by the photoresist of the resist pattern 71, is removed by RIE (Reactive Ion Etching) or the like. The RIE may use, as an etching gas, a gas including a chlorine component.

Next, as illustrated in FIG. 37A, the resist pattern 71 is removed using an organic solvent or the like, to form a resist pattern 72 having openings 72 a and 72 b in the regions where the source electrode 32 and the drain electrode 33 are to be formed. More particularly, after removing the resist pattern 71 using the organic solvent or the like, a photoresist is again coated on the electron supply layer 23, and the photoresist is exposed by the exposure apparatus and developed. Hence, the resist pattern 72, having the openings 72 a and 72 b in the regions where the source electrode 32 and the drain electrode 33 are to be formed, is formed. As illustrated in FIG. 37A, the resist pattern 72 is formed by stacking 2 resist layers.

Next, as illustrated in FIG. 37B, a metal multilayer 81 made of Ti/Al is deposited on the surface of the stacked structure formed with the resist pattern 72, by vacuum deposition. More particularly, a Ti layer is deposited on the surface of the stacked structure formed with the resist pattern 72, by vacuum deposition, and an Al layer is thereafter deposited on the Ti layer by vacuum deposition. In this embodiment, the Ti layer is deposited to a thickness of approximately 20 nm, and the Al layer is deposited to a thickness of approximately 200 nm.

Next, as illustrated in FIG. 37C, the metal multilayer 81 formed on the resist pattern 72 is removed together with the resist pattern 72 by lift-off, by dipping the stacked structure in an organic solvent or the like. As a result, the source electrode 32 and the drain electrode 33 are formed by the metal multilayer 81 remaining within the regions where the openings 72 a and 72 b of the resist pattern 72 were formed. Thereafter, a thermal process is performed at a temperature of 550° C., in order to establish an ohmic contact between the electron supply layer 23 and each of the source electrode 32 and the drain electrode 33.

Next, as illustrated in FIG. 38A, an insulator layer (hereinafter also referred to as “a hard mask insulator layer”) 73 for forming a hard mask which will be described later is formed on the electron supply layer 23, the source electrode 32, and the drain electrode 33. More particularly, an SiN layer having a thickness of approximately 20 nm is deposited on the electron supply layer 23, the source electrode 32, and the drain electrode 33 by plasma CVD (Chemical Vapor Deposition), to form the hard mask insulator layer 73. When forming the hard mask insulator layer 73 by the plasma CVD, silane, ammonia, or the like may be used for a source gas, for example. A refractive index of the hard mask insulator layer 73 at a wavelength of 633 nm is 2.0, and the hard mask insulator layer 73 is a stoichiometric layer.

Next, as illustrated in FIG. 38B, a resist pattern 74 having an opening 74 a is formed on the hard mask insulator layer 73. More particularly, a photoresist is coated on the hard mask insulator layer 73, and the resist pattern 74 having the opening 74 a is formed by exposing the photoresist by the exposure apparatus and developing the exposed photoresist. The opening 74 a of the resist pattern 74 has a width of approximately 0.5 μm, and has a size identical to the size of the oxide layer 50 that is formed by oxidizing the electron supply layer 23, as will be described later in the specification.

Next, as illustrated in FIG. 38C, an opening 73 a is formed by removing the hard mask insulator layer 73 exposed within the opening 74 a of the resist pattern 74 by RIE or the like. As a result, a hard mask 73 b having the opening 73 a is formed by the remaining hard mask insulator layer 73. The resist pattern 74 is thereafter removed using an organic solvent or the like.

Next, as illustrated in FIG. 39A, the oxide layer 50 is formed by oxidizing the surface of the electron supply layer 23 exposed within the opening 73 a of the hard mask 73 b, by the H₂O oxidation using water vapor. More particularly, an ALD (Atomic Layer Deposition) apparatus is used to form the oxide layer 50 by setting the substrate temperature to 300° C., supplying water vapor (H₂O) which becomes an oxidation source, and oxidizing the surface of the electron supply layer 23 exposed within the opening 73 a of the hard mask 73 b. The insulator layer 50 that is formed in this manner has a thickness of approximately 1.5 nm, and a width D1 of approximately 0.5 μm.

Next, as illustrated in FIG. 39B, a resist pattern 75 is formed on the oxide layer 50. More particularly, a photoresist is formed on the electron supply layer 23, the oxide layer 50, the source electrode 32, and the drain electrode 33, and the resist pattern 75 is formed by exposing the photoresist by the exposure apparatus and developing the exposure photoresist.

Next, as illustrated in FIG. 39C, the hard mask 73 b in regions where no resist pattern 75 is formed is removed using a buffered hydrogen fluoride or the like. Thereafter, the resist pattern 75 is removed using an organic solvent or the like.

Next, as illustrated in FIG. 40A, an insulator layer 60, which becomes a gate insulator layer, is formed on the electron supply layer 23, the oxide layer 50, the source electrode 32, and the drain electrode 33. More particularly, an Al₂O₃ layer having a thickness of approximately 2 nm is deposited on the electron supply layer 23, the oxide layer 50, the source electrode 32, and the drain electrode 33 by ALD (Atomic Layer Deposition), to form the insulator layer 60 that becomes the gate insulator layer. When forming the insulator layer 60 by the ALD, TMA (Trimethylaluminium), H₂O, or the like may be used for a source gas, for example, and the deposition may be performed at a substrate temperature of 300° C., for example.

Next, as illustrated in FIG. 40B, a resist pattern 77 for forming the gate electrode 31 is formed on the insulator layer 60. This resist pattern 77 is formed by 3 electron beam resist layers that are stacked, and has an opening 77 a in the region where the gate electrode 31 is to be formed. More particularly, the 3 electron beam resist layers are successively formed on the insulator layer 60 by repeatedly coating the same, for example. In addition, exposing the electron beam resist by the electron beam lithography apparatus and developing the exposed electron beam resist are repeated for each of the 3 electron beam resist layers, in order to form the opening 77 a in the 3 electron beam resist layers. As a result, the opening 77 a is formed in the resist pattern 77. The opening 77 a of the resist pattern 77 is formed to have a width of 0.8 μm, 1.3 μm, and 0.2 μm for the top, middle, and bottom electron beam resist layers forming the 3 electron beam resist layers.

Next, as illustrated in FIG. 40C, a metal multilayer 82 made of Ni/Au is deposited on the surface of the stacked structure formed with the resist pattern 77, by vacuum deposition. More particularly, an Ni layer is deposited on the surface of the stacked structure formed with the resist pattern 77, by vacuum deposition, and an Au layer is thereafter deposited on the Ni layer by vacuum deposition. In this embodiment, the Ni layer is deposited to a thickness of approximately 10 nm, and the Au layer is deposited to a thickness of approximately 300 nm.

Next, as illustrated in FIG. 41, the metal multilayer 82 formed on the resist pattern 77 is removed together with the resist pattern 77 by lift-off, by dipping the stacked structure in an organic solvent or the like. As a result, the gate electrode 31 is formed by the metal multilayer 82 remaining within the region where the opening 77 a of the resist pattern 77, on the insulator layer 60, was formed.

The semiconductor device in this embodiment is manufactured by the processes described above.

Fifth Embodiment

Next, a description will be given of the semiconductor device in a fifth embodiment. In this embodiment, the semiconductor device has the same structure as the semiconductor device in the fourth embodiment, however, the H₂O oxidation using water vapor, performed to form the oxide layer 50, is performed for a time that is two times longer than that used in the fourth embodiment. Hence, the thickness of the oxide layer 50 formed in this fifth embodiment is approximately 3.0 nm. In other words, the thickness of the oxide layer 50 of the semiconductor device in the fifth embodiment is two times the thickness of the oxide layer 50 of the semiconductor device in the fourth embodiment.

When the H₂O oxidation using water vapor is performed, the oxidation gradually progresses from the surface of the electron supply layer 23. Hence, the In composition ratio at the surface of the oxide layer 50 of the semiconductor device in this fifth embodiment becomes lower than that of the semiconductor device in the fourth embodiment. In other words, the Al composition ratio at the surface of the oxide layer 50 of the semiconductor device in this fifth embodiment becomes higher than that of the semiconductor device in the fourth embodiment, and the proportion of Al₂O₃ in this fifth embodiment is higher than that of the fourth embodiment. In this fifth embodiment in which an Al₂O₃ layer is formed as the insulator layer 60 on the oxide layer 50 having the high proportion of Al₂O₃, it is possible to further reduce the electrodes trapped at the interface between the oxide layer 50 and the insulator layer 60 when compared to the fourth embodiment. As a result, this fifth embodiment can further suppress the generation of the current collapse.

(Properties of Semiconductor Device)

Next, a description will be given of the properties of the semiconductor device in this embodiment. FIG. 42 illustrates measured results of the gate leak current that flows when a voltage is applied across the gate electrode and the drain electrode of the semiconductor device in this embodiment. In FIG. 42, the abscissa indicates the applied voltage, and the ordinate indicates the gate leak current. FIG. 42 is a correlation diagram of the gate voltage and the leak current of the semiconductor device in the fifth embodiment.

According to the semiconductor device in this embodiment, no gate leak current flows even when the voltage of up to 16 V is applied across the gate electrode and the drain electrode, as illustrated in FIG. 42. On the other hand, according to the semiconductor device illustrated in FIG. 1, the gate leak current flows when the voltage is applied across the gate electrode and the drain electrode, as illustrated in FIG. 33. Hence, it is confirmed that the semiconductor device in this embodiment can suppress the generation of the gate leak current. The generation of the gate leak current is suppressed and the gate leak current can be reduced according to the semiconductor device in this embodiment, because the oxide layer 50 and the insulator layer 60 are formed immediately below the gate electrode 31.

Next, a description will be given of the current collapse of the semiconductor device in this embodiment, in comparison with the current collapse of the semiconductor device illustrated in FIG. 27. FIG. 43 illustrates a relationship between the drain voltage V_(ds) and the drain current I_(ds) when the gate voltage V_(gs) is varied, for a case in which the drain voltage V_(ds) is increased up to 10 V, and for a case in which the drain voltage V_(ds) is increased up to 20 V. FIG. 43 is a correlation diagram of the drain voltage and the drain current of the semiconductor device in the fifth embodiment.

According to the semiconductor device having the structure illustrated in FIG. 27, the drain current I_(ds) decreases due to the current collapse in the case in which the drain voltage V_(ds) is increased up to 20 V, as illustrated in FIG. 35. On the other hand, according to the semiconductor device in this embodiment, the drain current I_(ds) does not decrease even in the case in which the drain voltage V_(ds) is increased up to 20 V, as illustrated in FIG. 43, and the generation of the current collapse is suppressed. In addition, when the properties of the semiconductor device in this embodiment illustrated in FIG. 43 and the properties of the semiconductor device in the fourth embodiment illustrated in FIG. 34 are compared, it is confirmed that the generation of the current collapse can be suppressed more in this embodiment than in the fourth embodiment. It may be regarded that this embodiment can further suppress the generation of the current collapse, even when compared to the fourth embodiment, because the oxidation time of InAlN by the H₂O oxidation using water vapor is longer than that of the fourth embodiment and the thickness of the oxidation layer 50 becomes greater than that of the fourth embodiment. In addition, it may be regarded that this embodiment can further suppress the generation of the current collapse, because the Al composition ratio at the surface of the oxide layer 50 in this embodiment is higher than that of the fourth embodiment.

Therefore, according to the semiconductor device in this embodiment, it is possible to reduce the gate leak current. In addition, because the generation of the current collapse can be suppressed, it is possible to suppress the ON-resistance from becoming high.

Other features of the fifth embodiment are the same as those of the fourth embodiment.

Sixth Embodiment

(Semiconductor Device)

Next, a description will be given of the semiconductor device in a sixth embodiment, by referring to FIG. 44. FIG. 44 is a structure diagram illustrating the semiconductor device in the sixth embodiment. The semiconductor device illustrated in FIG. 44 includes a buffer layer (not illustrated), an i-GaN electron transit layer 21, an AlN intermediate layer 22, and an InAlN electron supply layer 23 that are stacked on a substrate 10. A source electrode 32 and a drain electrode 33 are provided on the electron supply layer 23. An oxide layer 150 is formed by oxidizing a material forming the electron supply layer 23, at a surface of the electron supply layer 23 immediately under a region where a gate electrode 31 is to be formed. An insulator layer 60 that becomes a gate insulator layer is provided on the electron supply layer 23 and the oxide layer 150. The insulator layer 60 is made of Al₂O₃ or the like. The gate electrode 31 is provided on the insulator layer 60 in a region where the oxide layer 150 is formed. In this embodiment, the substrate 10 is made of semi-insulating SiC, and a 2DEG 21 a is generated in the electron transit layer 21 at the vicinity of the interface between the electron transit layer 21 and the intermediate layer 22.

The oxide layer 150 of the semiconductor device in this embodiment is formed by oxidizing the electron supply layer 23. The oxide layer 150 is not only formed in a region directly under the gate electrode 31, but is also formed in a region from the region directly under the gate electrode 31 to an intermediate part between the gate electrode 31 and the drain electrode 33. Hence, it is possible to further improve the insulation provided by the oxide layer 150, and obtain a high output from the semiconductor device.

Because the oxide layer 150 is formed by oxidizing the electron supply layer 23, thickness of the electron supply layer 23 in the region where the oxide layer 150 is formed is smaller than the thickness of the electron supply layer 23 in the region where no oxide layer 150 is formed. When the thickness of the electron supply layer 23 becomes smaller in the region where the oxide layer 150 is formed, the 2DEG 21 a directly under this region decreases. In the case of the semiconductor device in this embodiment having the electron transit layer 21 formed by i-GaN and the electron supply layer 23 formed by InAlN, the 2DEG 21 a having a high concentration is generated in the electron transit layer 21 at the vicinity of the interface between the electron transit layer 21 and the intermediate layer 22. In the case in which the 2DEG 21 a in the electron transit layer 21 has the high concentration, it is possible to increase the drain current, however, a gate breakdown voltage and an off-state breakdown voltage decrease.

Accordingly, in this embodiment, the oxide layer 150 is formed in the region from directly under the gate electrode 31 to the intermediate part between the gate electrode 31 and the drain electrode 33, in order to reduce the thickness of the electron supply layer 23 in the region where the oxide layer 150 is formed. Hence, it is possible to decrease the density of the 2DEG 21 a that is generated in the region from directly under the gate electrode 31 to the intermediate part between the gate electrode 31 and the drain electrode 33. By decreasing the density of the 2DEG 21 a in the region from directly under the gate electrode 31 to the intermediate part between the gate electrode 31 and the drain electrode 33, it becomes possible to improve the gate breakdown voltage and the off-state breakdown voltage, and obtain the high output from the semiconductor device. In addition, as illustrated in FIG. 45, a depletion layer region 151 can be extended to a desired region, in order to improve a high-frequency response of the semiconductor device.

FIG. 45 is a diagram for explaining the semiconductor device in the sixth embodiment. In FIG. 45, a two-dot chain line indicates a spread of the depletion layer region 151 when a voltage is applied to the gate electrode 31. As illustrated in FIG. 45, the depletion layer region 151 can easily be extended between the gate electrode 31 and the drain electrode 33, to the region in which the 2DEG 21 a is generated.

In this embodiment, a thickness T1 of the electron supply layer 23 in the region where no oxide layer 150 is formed is 10 nm, and in a case in which the thickness of the oxide layer 150 is 3 nm, a thickness T2 of the electron supply layer 23 in the region where the oxide layer 150 is formed is 7 nm.

(Properties of Semiconductor Device)

Next, a description will be given of the properties of the semiconductor device in this embodiment. FIG. 46 illustrates measured results of the gate leak current that flows when a voltage is applied across the gate electrode and the drain electrode of the semiconductor device in this embodiment. In FIG. 46, the abscissa indicates the applied voltage, and the ordinate indicates the gate leak current. FIG. 46 is a correlation diagram of the gate voltage and the leak current of the semiconductor device in the fifth embodiment.

According to the semiconductor device in this embodiment, no gate leak current flows even when the voltage of up to 16 V is applied across the gate electrode and the drain electrode, as illustrated in FIG. 46. On the other hand, according to the semiconductor device illustrated in FIG. 1, the gate leak current flows when the voltage is applied across the gate electrode and the drain electrode, as illustrated in FIG. 33. Hence, it is confirmed that the semiconductor device in this embodiment can suppress the generation of the gate leak current. The generation of the gate leak current is suppressed and the gate leak current can be reduced according to the semiconductor device in this embodiment, because the oxide layer 150 and the insulator layer 60 are formed immediately below the gate electrode 31.

Next, a description will be given of properties of the semiconductor device in the fourth embodiment and the semiconductor device in the sixth embodiment, by referring to FIG. 47. FIG. 47 is a diagram for explaining the properties of the semiconductor devices in the fourth embodiment and the sixth embodiment. As illustrated in FIG. 47, a gate-source capacitance C_(gs) is 500 (fF/mm) and the same for the semiconductor device in the fourth embodiment and the semiconductor device in the sixth embodiment. In addition, a drain-source capacitance C_(ds) is 150 (fF/mm) and the same for the semiconductor device in the fourth embodiment and the semiconductor device in the sixth embodiment.

On the other hand, a gate-drain capacitance C_(gd) is 130 (fF/mm) for the semiconductor device in the fourth embodiment, but is 110 (fF/mm) for the semiconductor device in the sixth embodiment. In other words, the gate-drain capacitance C_(gd) of the semiconductor device in the sixth embodiment is lower than that of the semiconductor device in the fourth embodiment.

In addition, a maximum transmission frequency f_(max) is 240 (GHz) for the semiconductor device in the fourth embodiment, but is 250 (GHz) for the semiconductor device in the sixth embodiment. In other words, the maximum transmission frequency f_(max) of the semiconductor device in the sixth embodiment is higher than that of the semiconductor device in the fourth embodiment. Accordingly, the frequency response of the semiconductor device in the sixth embodiment can further be improved compared to that of the semiconductor device in the fourth embodiment.

(Method of Manufacturing Semiconductor Device)

Next, a description will be given of the method of manufacturing the semiconductor device in the sixth embodiment, by referring to FIGS. 48A through 53. FIGS. 48A through 53 are diagrams for explaining processes of the method of manufacturing the semiconductor device in this embodiment.

First, as illustrated in FIG. 48A, the buffer layer (not illustrated), the electron transit layer 21, the intermediate layer 22, and the electron supply layer 23 are expitaxially grown on the substrate 10 by MOVPE and successively stacked. In this embodiment, each of the buffer layer (not illustrated), the electron transit layer 21, the intermediate layer 22, and the electron supply layer 23 may also be referred to as “a nitride semiconductor layer”. The electron transit layer 21 is formed by i-GaN to a thickness of approximately 3 μm, the intermediate layer 22 is formed by i-AlN to a thickness of approximately 1 nm, and the electron supply layer 23 is formed by i-In_(0.17)Al_(0.83)N to a thickness of approximately 10 nm. Hence, the 2DEG 21 a is generated in the electron transit layer 21 at the vicinity of the interface between the electron transit layer 21 and the intermediate layer 22. The substrate 10 is formed by semi-insulating SiC, and the buffer layer (not illustrated) is formed by GaN, AlGaN, or the like.

Next, as illustrated in FIG. 48B, an isolation region 61 is formed in the nitride semiconductor layers that are formed on the substrate 10. More particularly, a photoresist is coated on the electron supply layer 23, and a resist pattern (not illustrated), having an opening in a region where the isolation region 61 is to be formed, is formed by exposing the photoresist by the exposure apparatus and developing the exposed photoresist. Thereafter, Ar ions or the like are injected into the nitride semiconductor layers within the opening of the resist pattern, in order to form the isolation region 61. When forming the isolation region 61, the Ar ions or the like may be injected into a part of the substrate 10. The resist pattern is thereafter removed using an organic solvent or the like.

Next, as illustrated in FIG. 48C, a resist pattern 71, having openings 71 a and 71 b in regions where the source electrode 32 and the drain electrode 33 are to be formed, is formed on the electron supply layer 23, and then, a part at the surface of the electron supply layer 23 is removed within the openings 71 a and 71 b. More particularly, a photoresist is coated on the electron supply layer 23, and the resist pattern 71, having the openings 71 a and 71 b in the regions where the source electrode 32 and the drain electrode 33 are to be formed, is formed by exposing the photoresist by the exposure apparatus and developing the exposed photoresist. Thereafter, the part at the surface of the electron supply layer 23, exposed within the openings 71 a and 71 b of the resist pattern 71 and not covered by the photoresist of the resist pattern 71, is removed by RIE or the like. The RIE may use, as an etching gas, a gas including a chlorine component.

Next, as illustrated in FIG. 49A, the resist pattern 71 is removed using an organic solvent or the like, to form a resist pattern 72 having openings 72 a and 72 b in the regions where the source electrode 32 and the drain electrode 33 are to be formed. More particularly, after removing the resist pattern 71 using the organic solvent or the like, a photoresist is again coated on the electron supply layer 23, and the photoresist is exposed by the exposure apparatus and developed. Hence, the resist pattern 72, having the openings 72 a and 72 b in the regions where the source electrode 32 and the drain electrode 33 are to be formed, is formed. As illustrated in FIG. 18A, the resist pattern 72 is formed by stacking 2 resist layers.

Next, as illustrated in FIG. 49B, a metal multilayer 81 made of Ti/Al is deposited on the surface of the stacked structure formed with the resist pattern 72, by vacuum deposition. More particularly, a Ti layer is deposited on the surface of the stacked structure formed with the resist pattern 72, by vacuum deposition, and an Al layer is thereafter deposited on the Ti layer by vacuum deposition. In this embodiment, the Ti layer is deposited to a thickness of approximately 20 nm, and the Al layer is deposited to a thickness of approximately 200 nm.

Next, as illustrated in FIG. 49C, the metal multilayer 81 formed on the resist pattern 72 is removed together with the resist pattern 72 by lift-off, by dipping the stacked structure in an organic solvent or the like. As a result, the source electrode 32 and the drain electrode 33 are formed by the metal multilayer 81 remaining within the regions where the openings 72 a and 72 b of the resist pattern 72 were formed. Thereafter, a thermal process is performed at a temperature of 550° C., in order to establish an ohmic contact between the electron supply layer 23 and each of the source electrode 32 and the drain electrode 33.

Next, as illustrated in FIG. 50A, an insulator layer (hereinafter also referred to as “a hard mask insulator layer”) 173 for forming a hard mask which will be described later is formed on the electron supply layer 23, the source electrode 32, and the drain electrode 33. More particularly, an SiN layer having a thickness of approximately 20 nm is deposited on the electron supply layer 23, the source electrode 32, and the drain electrode 33 by plasma CVD, to form the hard mask insulator layer 173. When forming the hard mask insulator layer 173 by the plasma CVD, silane, ammonia, or the like may be used for a source gas, for example. A refractive index of the hard mask insulator layer 173 at the wavelength of 633 nm is 2.0, and the hard mask insulator layer 173 is a stoichiometric layer.

Next, as illustrated in FIG. 50B, a resist pattern 174 having an opening 174 a is formed on the hard mask insulator layer 173. More particularly, a photoresist is coated on the hard mask insulator layer 173, and the resist pattern 174 having the opening 174 a is formed by exposing the photoresist by the exposure apparatus and developing the exposed photoresist. The opening 174 a of the resist pattern 174 has a width of approximately 1.0 μm, and has a size identical to the size of the oxide layer 150 that is formed by oxidizing the electron supply layer 23, as will be described later in the specification. In this embodiment, the oxide layer 150 partially extends from a region directly under the gate electrode 31 towards the drain electrode 33.

Next, as illustrated in FIG. 50C, an opening 173 a is formed by removing the hard mask insulator layer 173 exposed within the opening 174 a of the resist pattern 174 by RIE or the like. As a result, a hard mask 173 b having the opening 173 a is formed by the remaining hard mask insulator layer 173. The resist pattern 174 is thereafter removed using an organic solvent or the like.

Next, as illustrated in FIG. 51A, the oxide layer 150 is formed by oxidizing the surface of the electron supply layer 23 exposed within the opening 173 a of the hard mask 173 b, by the H₂O oxidation using water vapor. More particularly, the ALD apparatus is used to form the oxide layer 150 by setting the substrate temperature to 300° C., supplying water vapor (H₂O) which becomes an oxidation source, and oxidizing the surface of the electron supply layer 23 exposed within the opening 173 a of the hard mask 173 b. The oxide layer 150 that is formed in this manner has a thickness of approximately 1.5 nm, and a width D3 of approximately 1.0 μm so as to partially extend from the region directly under the gate electrode 31 towards the drain electrode 33.

Next, as illustrated in FIG. 51B, a resist pattern 175 is formed on the oxide layer 150. More particularly, a photoresist is formed on the electron supply layer 23, the oxide layer 150, the source electrode 32, and the drain electrode 33, and the resist pattern 175 is formed by exposing the photoresist by the exposure apparatus and developing the exposure photoresist.

Next, as illustrated in FIG. 51C, the hard mask 173 b in regions where no resist pattern 175 is formed is removed using a buffered hydrogen fluoride or the like. Thereafter, the resist pattern 175 is removed using an organic solvent or the like.

Next, as illustrated in FIG. 52A, an insulator layer 60, which becomes a gate insulator layer, is formed on the electron supply layer 23, the oxide layer 150, the source electrode 32, and the drain electrode 33. More particularly, an Al₂O₃ layer having a thickness of approximately 2 nm is deposited on the electron supply layer 23, the oxide layer 150, the source electrode 32, and the drain electrode 33 by ALD, to form the insulator layer 60 that becomes the gate insulator layer. When forming the insulator layer 60 by the ALD, TMA, H₂O, or the like may be used for a source gas, for example, and the deposition may be performed at a substrate temperature of 300° C., for example.

Next, as illustrated in FIG. 52B, a resist pattern 77 for forming the gate electrode 31 is formed on the insulator layer 60. This resist pattern 77 is formed by 3 electron beam resist layers that are stacked, and has an opening 77 a in the region where the gate electrode 31 is to be formed. More particularly, the 3 electron beam resist layers are successively formed on the insulator layer 60 by repeatedly coating the same, for example. In addition, exposing the electron beam resist by the electron beam lithography apparatus and developing the exposed electron beam resist are repeated for each of the 3 electron beam resist layers, in order to form the opening 77 a in the 3 electron beam resist layers. As a result, the opening 77 a is formed in the resist pattern 77. The opening 77 a of the resist pattern 77 is formed to have a width of 0.8 μm, 1.3 μm, and 0.2 μm for the top, middle, and bottom electron beam resist layers forming the 3 electron beam resist layers.

Next, as illustrated in FIG. 52C, a metal multilayer 82 made of Ni/Au is deposited on the surface of the stacked structure formed with the resist pattern 77, by vacuum deposition. More particularly, an Ni layer is deposited on the surface of the stacked structure formed with the resist pattern 77, by vacuum deposition, and an Au layer is thereafter deposited on the Ni layer by vacuum deposition. In this embodiment, the Ni layer is deposited to a thickness of approximately 10 nm, and the Au layer is deposited to a thickness of approximately 300 nm.

Next, as illustrated in FIG. 53, the metal multilayer 82 formed on the resist pattern 77 is removed together with the resist pattern 77 by lift-off, by dipping the stacked structure in an organic solvent or the like. As a result, the gate electrode 31 is formed by the metal multilayer 82 remaining within the region where the opening 77 a of the resist pattern 77 was formed.

The semiconductor device in this embodiment is manufactured by the processes described above.

Other features of the sixth embodiment are the same as those of the fourth embodiment.

Seventh Embodiment

Next, a description will be given of a seventh embodiment. In this embodiment, the present invention is applied to a semiconductor device, a power supply unit, and a high-output amplifier (or high-frequency amplifier).

(Semiconductor Device)

The semiconductor device in this embodiment includes a discrete package of the semiconductor device in any of the fourth through sixth embodiments described above. A description of the discrete package of the semiconductor device may be the same as that described above in conjunction with FIG. 23, and a description thereof will be omitted.

(PFC Circuit, Power Supply Unit & High-Output Amplifier)

Next, a description will be given of the PFC circuit, the power supply unit, and the high-output amplifier (or high-frequency amplifier) in this embodiment. The PFC circuit, the power supply unit, and the high-output amplifier (or high-frequency amplifier) in this embodiment respectively are the power supply unit and the high-output amplifier (or high-frequency amplifier) using the semiconductor device in any of the fourth through sixth embodiments.

(PFC Circuit)

The PFC circuit in this embodiment may be the same as that described above in conjunction with FIG. 24, and a description thereof will be omitted.

(Power Supply Unit)

The power supply unit in this embodiment may be the same as that described above in conjunction with FIG. 25, and a description thereof will be omitted.

(High-Output Amplifier)

The high-output amplifier (or high-frequency amplifier) in this embodiment may be the same as that described above in conjunction with FIG. 26, and a description thereof will be omitted.

In each of the embodiments described above, the source electrode 32 and the drain electrode 33 may be formed on the electron transit layer 21 forming the first semiconductor layer, or on the electron supply layer 23 forming the second semiconductor layer.

According to each of the embodiments described above, it is possible to reduce the gate leak current in the HEMT that uses InAlN for the electron supply layer and has the Schottky gate electrode.

Although the embodiments are numbered with, for example, “first,” “second,” . . . “seventh,” the ordinal numbers do not imply priorities of the embodiments. Many other variations and modifications will be apparent to those skilled in the art.

All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention. 

What is claimed is:
 1. A semiconductor device comprising: a first semiconductor layer made of a nitride semiconductor and formed on a substrate; a second semiconductor layer made of a material including InAlN and formed on the first semiconductor layer; a first insulator layer formed by an oxidized surface part of the second semiconductor layer; a gate electrode formed on the first insulator layer; and a source electrode and a drain electrode respectively formed on the second semiconductor layer, wherein the first insulator layer includes In—O, and more Al—O than the In—O.
 2. The semiconductor device as claimed in claim 1, wherein the oxidized surface part forming the first insulator layer is formed by oxidation using water vapor.
 3. The semiconductor device as claimed in claim 1, wherein the first insulator layer extends from a region directly under the gate electrode towards the drain electrode to an intermediate part between the gate electrode and the drain electrode.
 4. The semiconductor device as claimed in claim 1, wherein the first insulator layer includes more Al—O than In—O when observed by XPS (X-ray Photoelectron Spectroscopy) analysis.
 5. The semiconductor device as claimed in claim 1, further comprising: a third semiconductor layer made of a nitride semiconductor and formed between the first semiconductor layer and the second semiconductor layer.
 6. The semiconductor device as claimed in claim 5, wherein the third semiconductor layer is made of a material including AlN.
 7. The semiconductor device as claimed in claim 1, wherein the first semiconductor layer is made of a material including GaN.
 8. The semiconductor device as claimed in claim 1, wherein the first insulator layer has a thickness of 1 nm or greater and 5 nm or less.
 9. The semiconductor device as claimed in claim 1, further comprising: a second insulator layer formed on the second semiconductor layer and the first insulator layer, wherein the gate electrode is formed on the second insulator layer in a region where the first insulator region is formed.
 10. The semiconductor device as claimed in claim 9, wherein the second insulator layer is made of a material selected from a group consisting of oxides, nitrides, and oxynitrides.
 11. The semiconductor device as claimed in claim 1, further comprising: a second insulator layer formed on the second semiconductor layer and the first insulator layer, and covering the first insulator layer in its entirety.
 12. The semiconductor device as claimed in claim 11, wherein the second insulator layer is made of a material selected from a group consisting of Al₂O₃, AlN, and AlON.
 13. A power supply unit comprising: a semiconductor device according to claim
 1. 14. An amplifier comprising: a semiconductor device according to claim
 1. 